Subject information acquiring devices which use X rays and ultrasounds are used in many fields which require nondestructive test, including a medical field. In the field of subject information acquiring devices in the medical, physiological information (that is, function information) about a biological body can be effectively used to find an ailing site such as cancer, and thus imaging of function information has been studied in recent years. A photoacoustic tomography (PAT) which is one of optical imaging techniques is proposed as one of diagnosing methods using function information. While X-ray diagnosis or ultrasound diagnosis can only acquire morphology information about the interior of the biological body, the photoacoustic tomography can acquire both of morphology and function information with non-invasive diagnosis.
In The photoacoustic tomography, pulsed light produced from a light source is radiated on an interior of a subject, and then an acoustic wave (typically, ultrasound) produced by a photoacoustic effect of internal tissues which absorbed light propagated and diffused in the subject is detected. The detected acoustic wave which contains information about the internal tissues which is the source of the acoustic wave is then converted into an image of the information. By detecting a temporal change of the received acoustic wave at a plurality of sites encircling the subject, and mathematically analyzing (reconstructing) the obtained signal, it is possible to three-dimensionally visualize information related to an optical characteristic value inside the subject. This information can be used as morphology information about the interior of the subject, and, further, function information including an optical characteristic value distribution such as an absorption coefficient distribution inside the subject can also be obtained from the initial acoustic pressure distribution produced by radiating light on the interior of the subject.
As the pulsed light to be radiated on the interior of the subject near-infrared light, for example, can be used. Near-infrared light has the property that it easily transmits through water which constitutes most part of the biological body, while it is easily absorbed by hemoglobin in blood, so that it is possible to image a blood vessel image as morphology information. Further, by using the absorption coefficient distribution obtained by radiating near-infrared light, it is possible to learn a content rate of oxygenated hemoglobin to all hemoglobin in blood, that is, it is possible to learn the oxygen saturation and, consequently, imaging of biological function can also be performed. The oxygen saturation distribution serves as an indicator to distinguish whether a tumor is benign or malignant, and hence photoacoustic tomography is expected as a way for efficiently finding malignant tumors.
The oxygen saturation is calculated by performing a plurality times of measurements using pulsed light of different wavelengths, and then performing a comparison operation of calculating the ratio of absorption coefficients calculated for different wavelengths. This is based on a principle that optical absorption spectra of deoxygenated hemoglobin and oxygenated hemoglobin are different. The content rate can be consequently found out by measuring and comparing the spectra according to the different wavelengths.
In those imaging processes, when the obtained absorption coefficients are directly used for the comparison operation of calculating the ratio, a blood vessel image portion and a background portion cannot be distinguished, and therefore, as disclosed in Non Patent Literature 1, it is necessary to distinguish between the blood vessel image portion and background portion and process only the blood vessel image portion.